During a 16 year program, a synthetic trileaflet valve had been developed. Prototype clinical valves use a narrow soft sewing collar of knitted polyster, a flexible coronal shaped stent and a micro-woven fabric which is highly flexible and has stiffness and anisotropic properties similar to normal aortic leaflet tissue. Extensive durability and soft tissue implant studies were performed prior to mitral valve replacement in sheep. The in vivo data showed that valvular insufficiency occurred early (24 hours) which has subsequently been traced to alteration of filament geometry from stress compaction. New fabric designs have been completed and a new prototype has been produced. Valves are inserted into a hydraulic testing device which characterizes forward and regurgitant power losses. Accelerated fatigue testing is performed at cyclic rates of 1000-1200/min. Implant studies consist of subcutaneous placement of materials in rabbits, conduit insertions in the arterial and venous systems of dogs and baboons and valve insertion in juvenile sheep.